Hydraulic system and periodically flushed throttle valve therefor



May 29, 1956 w. c. CRITCHLEY 2,747,371

HYDRAULIC SYSTEM AND PERIODICALLY FLUSHED THROTTLE VALVE THEREFOR Filed Nov. 15, 1952 20,21 FIG 2 47 45 43 494648 3329/393441 5g I I\ v 32 422 y 26 3| 5 2 g 28 35 5 INVENTOR.

25 44 3 WILLIAM c. CRITCHLEY ATTORNEY United States Patent HYDRAULIC SYSTEM AND PEREHDICAILLY FLUSHED THRGTTLE VALVE TR LREFQR William C. Critchley, Brighton, N. Y., assignor to The Gleason Works, Rochester, N. Y., a corporation of New York Application November 13, 1952, Serial No. 320,302

17 Claims. (Cl. 60-52) The present invention relates to a hydraulic system, such. as for operating the dresser for an abrasive. wheel of a, gear grinder, and an adjustable throttle valve there for, in which a means is provided for periodically flush.- ing; the valve.

The system includes an hydraulically operated motor controlled by a reversing valve, and ath-rottle valve arranged to govern the speed of themotor by restricting the flow of hydraulic fluid during operation in one direction. Invorder to effect such restriction, the throat of the throttle valve is very narrow and hence. is subject to clogging by solid particles of matter which may find their way into the hydraulic fluid. The invention' provides a means, operated automatically upon operation of the. motor in the reverse direction, to widen the throat of the. throttle so that any such matter lodged in the throat will be flushed away. To this end the throat of the throttle valve is formed between one end face of a: piston and an adjacent Wall portion of the valve cham her, and provision is made for reversing the pressure differential acting on the piston to cause it to move away from such wall portion and thereby release any particles lodged in the throat and cause them to be fiushed out of the valve chamber. Preferably the chamber wall portion referred tocomprises the inner end face. of a member that is adjustable relative to the throttle valve casing in a direction toward and away from the: end faceof the piston. in this way the width of the throttling throat may be adjusted to provide the desired motor speed.

The.- foregoing and other objects and advantages of the invention Will appear from the following description made with reference to the: drawing. In the latter:

Fig; 1 isa diagrammatic view of the system;

Fig. 2 is a longitudinal sectional view through the throttle valve; and

Fig. 3 is a section taken inthe plane indicated by line 3-3 of Fig. 2.

The systemshovvntins Fig. 1 lIlGljliClBS-EL? motor'comprisingcat piston 10 movable in a cylinder 11 by fluid drawn from a sump:- 12-and delivered under pressure by a pump 13*. The: system and the motor may be of various forms and arrangements, depending upon the use to which they areto be put, and the particular arrangement shown. is for the purpose. of operating the dresser for the abrasive wheel: of a gear. grinding, machine. For thispurpose the piston. 1-0 has rack teeth meshing with a pinion 14: secured to aashaft 15.- that is journaled forangular oscillation. in the cylinderblock. On. one end. of the shaft is. an. arm 161earr=ying a diamond or other dressing element 17 adapted; to pass. over. thesurface 18 of a cup-shaped abrasive wheel todress the surface. as the arm is swungby the Piston;

The. system includes-areversing valve 19 for controlling the motor10, 11-;a pair of throttle valves 20 and 21 ,-one of. which. controls.- the speed of the piston 10 during one stroke thereof; a: selector valve 22 whose position determines which one off the tlirottle valves is effective;

' a check valve 23' toenforce flow through the selected throttle valve; anda phase reversing valve 24, for determining the stroke of the piston 10 during which the throttling is. effective.

Each of valves 19 and 22 has a central slide member movableaxially (vertically in Fig. 1) in a cylindrical chamber through whose side wall extend the several ports controlled by the valve. The valve slide members may be operated either manually or automatically, the structure of these valves and the operating means therefor not being. part of the present invention. The same is true of the phase reversing valve 24 whose central. slide is movable axially (horizontally in Fig. 1).

The two throttle valves. 20 and 21 may both be of the same construction. As. shown in Fig. 2. each comprises a casing body 25 (if desired the two valves 20 and 21 may have. a. common casing) having. a central cylindrical bore 26- closed at one end by a head 27 and at the opposite end by a plug 28,. boththe head and the plug being se cured to. the casing body by suitable fasteners, not shown. The plug 2& has a central bore in which a pin 29 is slidable, and also has a plurality of circularly arranged fingers 30 adapted to act as stops to limit the motion of a. piston 31 that is slidable in the bore 26. The pin 29 has a plane. face 32 at its. inner end, an annular groove 33 containing. a seal to prevent the flow thereby of hydraulic fluid, and at itsouter end a keyway 34 anda reduced diameter screw-threaded portion Secured to the casing around the: outer end of pin 29 are inner and outer members 36. and 37 spaced by flange 38 of .member 36. Member 36 has a bore aligned and passing the. pin 29 and member 37 has an aligned semi-cylindrical recess. A key 39 secured to member 36 seats in keyway 34. to hold the pin 29 against rotation while permitting. it to move axially. A-n. adjusting. knob 40. has a knurled gripping portion partially disposed in the semi-cylindrical recess of member 37 and an enlarged head 41- whose outer conical surface may con tain suitable calibrations registering with markings on the outer face of member 36. The knob is screw-threaded to the. portion of the pin 29. A recess in" the rear face of. the knob contains a coil spring 42 compressedbetween the. knob and the front face of. member 36; In effect the inner end of pin 29, having end face 32, constitutes a wall portion of the inner chamber 46 of the valve.

The piston 31 has a central annular recess 43 register-'- ing. with a port 44 in the casingZS", and annular recesses 45 and 46 at its outer and inner ends, the latter recesses constituting parts of chambers in the casing which com. municate respectively with ports. 47 and- 48 formed in: the casing. The piston has a central opening 49- which communicates with the annular recess 43: and also: opens through the inner face of the piston at the centen thereof. Press-fitted into this opening is the tubular stem-p01 tion of. a valve face member: 50; This member, which constitutes a part of the piston, has apla-ne frontface 51 opposed to face 32 0f pin 29' and a central aperture 52 of substantially smaller diameter than the interior of the tubularstem portion and opening 49.

When the valve 20 or 21 is functioning as a throttle,

the piston 31 thereof ismoved inwardly (.to the. right in Fig. 2) to its limit position against stop fingers 30, by fluid pressure introduced into chamber 45 through port. 47.. The fluid being throttled. enters. chamber 46 through port 48, passes between surfaces-32and-51,1which constitute the throttle throat, and exhausts through aper ture. 52, opening 49; annular recess 43 and port 44.. The width. of. the throttle throat, While adjustable. as desired by turning the knob 40 to thereby advance. or, retract the pin 29 relative tothe valve face member 50; is ordinarily much less than the diameter of the aperture 52 so that any foreign particles in. the hydraulic fluid Q? will lodge in the throat rather than in the aperture. The spring 42 by pressing the head 41 of knob 40 against the innerface of member 37 prevents unintentional movement of the knob and of member 29.

When the pressure in chamber 45 is reduced below that prevailing in chamber 46 the piston 31 is moved (to the left inFig. 2) away from pin 29 to its limit position abutting head 27. This greatly increases the width of the throat between surfaces 32 and 51, nullifying the throttling effect thereof. This results in fluid flow at much higher velocity from chamber 46 through the throat and aperture 52 into opening 49, recess 43 and port 44, and serves to flush out of the throat any foreign particles that may have lodged therein during the preceding throttle action of the valve.

The operation of the system will now be described. With valves 22 and 24 in the positions shown in Fig. 1, and motor reversing valve 19 moved to its upper position shown, fluid under pressure from pump 13 will be directed to the chamber 45 of both throttle valves via line 53, valve 19, and lines 54 and 55, the terminal branches of the latter opening into ports 47 of the throttle valves. As ports 44 of the latter are on exhaust to the sump 12 through a return line 56, the throttle valve pistons 31 are moved to throttling position. Pressure from pump 13 is also applied via lines 54 and 57, valve 24, and line 58 to the right end of piston 10, moving the latter to the left, and thereby causing the dressing element 17 to be swung to the right over surface 18 of the abrasive wheel. This stroke of the piston 10, which is the wheel dressing stroke, is at a rate controlled by throttle valve 21, since fluid displaced from the left chamber of cylinder 11 exhausts to the sump through line 59, valve 24, lines 60 and 61 (check valve 23 now being closed), valve 22, line 62, the throttling passage through valve 21 (i. e. port 48, chamber 46, throat 32, 51, aperture 52, opening 49 and port 44), and line 56.

Upon reversal of valve 19 to its lowermost position, pressure from pump 13 and line 53 is applied to the left chamber of cylinder 11 by way of line 63, now open check valve 23, line 68, valve 24 and line 59.

The right chamber of the cylinder 11 is now on unthrottled exhaust, via line 58, valve 24, line 57 and 54, valve 19 and line 64 to the sump 12. Accordingly the dresser element 17 is moved rapidly to the left across the face 18 of the wheel, and has substantially no dressing effect. Simultaneously pump pressure is applied through line 63, now open check valve 23, line 61, valve 22, and line 62 to the chamber 46 of valve 21. This moves piston 31 to flushing position since the chamber 45 is on exhaust via lines 55 and 54, valve 19 and line 64 to sump 12; and effects the flushing of the throttle throat hereinbefore described.

It will be understood that by reversing the position of valve 22, line 61 will be connected to a line 65 extending to port 48 of valve 20, instead of to line 62 extending to the corresponding port of valve 21. Hence,

tion, and that various changes and modifications may be made without departing from the spirit of the invention or from the scope of the appended claims.

I'claim: 1. A throttle valve comprising a casing having a piston movable therein, the casing having a chamber adjacent each of the two end faces of the piston and a port opening into each chamber, the piston having a passage therethrough extending from one end face thereof to a port in the casing other than the aforementioned ports, the chamber adjacent said one end face of the piston having a wall portion which together with said one end face of the piston forms a throttling throat communicating with said passage, a stop for limiting motion of the piston toward said wall portion, and means for adjusting the relative positions of said wall portion and said stop to thereby vary the width of the throat.

2. A throttle valve according to claim 1 in which the adjusting means comprises a member adjustable relative to the casing toward and away from the piston, the inner end of the member constituting the wall portion of the chamber which together with said one end face of the piston forms the throttling throat.

3. A throttle valve according to claim 1 in which the adjusting means comprises a member slidable within the casing in the same direction as the piston, the inner end of said member constituting the wall portion of the chamber which together with said one end face of the piston forms the throttling throat, an adjusting element screw-threaded to said member and confined against axial motion relative to the casing, and means for holding said adjusting element against rotation relative to the casing.

4. A hydraulic system comprising a hydraulic motor and a reversing valve to alternately connect the opposite sides of the motor respectively to sources of fluid at relatively higher and lower pressures, to thereby alternately reverse the motion of the motor; a throttle valve having a casing, a piston movable in the casing, the casing having a chamber adjacent each of the two end faces of the piston, the piston having a passage therethrough extending from one end face thereof to a first port in the casing, the casing having second and third ports opening respectively into the chamber adjacent said one end face of the piston and the chamber adjacent the opposite end face of the piston, the first-mentioned chamber having a wall portion which together with the adjacent end face of the piston forms a throttling throat communicating with said passage, a stop for limiting motion of the piston toward said wall portion, and means for adjusting the relative positions of said wall portion and said stop to thereby vary the width of the throat; means for conducting exhaust fluid from the motor through the throttle valve, between the first and second ports, in one position of the reversing valve; and means for applying through the third port a fluid pressure sufficient to hold the piston against the stop When the reversing valve is in said one position, and for applying through the third port a pressure low enough to cause the piston to be held away from the stop when the reversing valve is in the other position thereof, to thereby effect flushing of the throat.

5. A hydraulic system comprising a hydraulic motor and a reversing valve to alternately connect the opposite sides of the motor respectively to sources of fluid at relatively higher and lower pressures, to thereby alternately reverse the motion of the motor; a throttle valve having a casing, a piston movable in the casing, the casing having a chamber adjacent each of the two end faces of the piston, the piston having a passage therethrough extending from one end face thereof to a first port in the casing, the casing having second and third ports opening respectively into the chamber adjacent said one end face of the piston and the chamber adjacent the opposite end face of the piston, the first-mentioned chamber having a wall portion which together with the adjacent end face of the piston forms a throttling throat communicating with said passage, and a stop for limiting motion of the piston toward said wall portion; means for conducting exhaust fluid from the motor through the throttle valve, between the first and second ports, in one position of the reversing valve; and means for applying through the third port a fluid pressure 5 suflicient tohold the piston against the stop when the reversing' valve is in said one position, and for-applying through the third port a pressure lowenough to cause the piston to be held away from the stop when the reversing valve is in the other position thereof, to thereby effect flushing of the throat.

6. A hydraulic system comprising a positive displacement motor, a pair of conduits connected to the opposite sides of the motor, a reversing valve for alternately connecting the conduits respectively to sources of fluid at relatively higher and lower pressures tothereby alternately reverse the motion of the motor; acheck valve in one of said conduits to prevent the flow therethrough of fluid exhausting from the motor; a throttle valve having a casing, a piston movable in the casing, the casing having a chamber adjacent each of the two end faces of the piston, the piston having a passage therethrough extendingfrom one end face thereof to a first port, in the casing, the casing having second and third ports opening respectively into the chamber adjacent said one. end face of the'piston and the chamber adjacent the opposite end face of the piston, the first-mentioned chamber having a wall portion which together with the adjacent end: face of the piston forms a throttling. throat communicating with said passage, a stop for limiting motion of the piston toward said wall portion, and means foradjusting the relative positions of said wall portion and said: stop to thereby vary the width. of the throat; a fluid connection between said one of said conduits, at a point in the conduit between the motor and the check valve, and one; of the first and second ports, a fluid connection between, the source of fluid. at lower pressure andthe other of said firstand second ports, and a fluid connection between the thirdport and the other one of said conduits.

7'. A hydraulic system. comprising a positive displace,- ment motor, a pair of conduits connected to the opposite sides of: the motor, a. reversing valve for alternately connecting the; conduits respectively to sources of fluid at relatively, higher and lower pressures, to thereby alternately reverse the motion of the motor; a check valve in one of said: conduits to prevent the flow therethrough of fluid exhausting from the motor; a throttle valvehaving a casing, a pistonmovable in the casing, the casing having a chamber adjacent, each of the two end facesofthe piston, thepistonhaving a passage therethrough extending from one; end: facethereof toa first port in the casing, the casing having, second andthird ports opening respectively into the chamber adjacent said one end face of the piston and the; chamber adjacent the opposite end face of the piston, thefirst-mentioned chamber having a wall. portion which together with the adjacent end face of the piston forms a throttling throat communicating with said. passage, and a stop for limiting motion of the piston toward said wall portion; a fluid connection between said one of said conduits, at a point in the conduit between the motor and the check valve, and one of the first and second ports, a fluid connection between the source of fluid at lower pressure andthe other of said first and second ports, and a I fluid connection between the third port and the other one of said conduits.

8. A hydraulic system comprising a sump, a pump for withdrawing fluid from the sump and discharging it under pressure, a positive displacement motor having a pair of conduits connected to the opposite sides thereof, a' reversing valve to connect either of said conduits to the pump discharge and the other thereof to the sump, and a check valve in one of said conduits to prevent the flow therethrough of. fluid exhausting from the motor; a throttle,valve having a casing, a piston movable in the casing, the casinghaving a chamber adjacent each of, the two end faces, of the piston, the piston having a passage therethrough extending from one end facethereof to a first por.t. in the casing, thecasing having second and third ports opening respectively into the chamber adjacent said one end face of the piston and the chamber adjacent the opposite end face of the piston, the first-mentioned chamher having a wall portion which together with the ad jacent end face of the piston terms a: throttling throat cornmunicating with said passage, a stop for limiting motion of the piston toward said wall portion, and means for adjusting the relative positions of said wall portion and said stop to thereby vary the width of the throat; a fluid connection between said one of said conduits, at a point in' the conduit between the motor and the check valve, and one of the first and second ports, a fluid connection between the sump and the other of said first and second ports, and a fluid connection between the third port and the other one ofsaid conduits.

9. A hydraulic system comprising a sump, a pump-for withdrawing fluid from thesump and discharging it under pressure, a positive displacement motor having a pair of conduits connected to the opposite sides thereof, a. 1'6? versing valve to connect either of said conduits; to the pump discharge and the other thereof to the sump, and a check valve in one of said conduits to prevent'the'flow therethrough of fluid exhausting from the motor;.;a throttle valve having acasing, a piston movable in the casing, the casing having'a chamber adjacent each of the two end faces of the piston, the piston having a' passage: there.- through extending from one end thereof to a first port in the casing, the casing having second and third ports opening respectively into the chamber adjacent said one end face of the piston and the chamber adjacent. the: opposi-te end face; of the piston, the first-mentioned chamber having a wall portion which together with the,- adjacent endface of the piston forms a throttling throat communieating with said passage, and a stop for limiting; motion of the piston toward said wall portion; a fluid connection between said one of said conduits, at a point in the conduit between: the motor and the check valve, and one; of the first and second ports, a fluid connection between the sumpand the other of said first and second ports, and'a fluid connection between the third port. andthe other one of said conduit-s.

10. A hydraulic system comprising a sump, apump for Withdrawing fluid, from the sump and discharging it under pressure, a positive displacement motor having; a pair of conduits connected to the opposite sides. thereof, a reversing valve to connect either ofsaid conduits, to the pump discharge and the other thereof to; the: sump, and a check valve in one of said: conduits to prevent therethrough of fluidi exhaustingv from the motor; a throttle valve having a casing, a piston movable: in; the casing, the; casing having a chamber adjacent each of the two end faces of the piston, the piston having a passage therethrough extending from one end face thereof to a first port in the casing, the casing having; second and third ports openingrespectively. intothe chamber adjacent said one end faceof the piston and the chamber adjacent the opposite endface, of the piston,,the first-mentioned chamber having a wall portion which together with the adjacent end face of the piston forms a throttling, throat communicating with said passage, a stop for limiting-motion of the piston toward said. wall portion, and means for adjusting the relative positions of. said: wall portion and said stop to thereby vary the. width of, the throat; a fluid connection between-saidv one of said conduits, at a point in the conduit between the motor and the. check valve, and the second port, afluid connection betweemthe sump and the first port, and a fluid connection between the third port and the other one of. saidconduits.

11. A hydraulic system comprisinga sump,,a .pump for withdrawing fluid from the sump anddischarging it under pressure, a positivedisplacement motor havingv a pair. of conduits connected to the opposite sides-.thereofla reversing valve to connecteither of saidv conduits to the pump discharge and the other thereof to the sump, and acheck valve in one of said conduits to" prevent the flow therethrough of fluid exhausting from the' motor; athrottle valve having a casing, a piston'movable-inthe casing the casing having a chamber adjacent each of the two end faces of the piston, the piston having a passage therethrough extending from one end face thereof to a first port in the casing, the casing having second and third ports opening respectively into the chamber adjacent said one end face of the piston and the chamber adjacent the opposite end face of the piston, a member adjustable relative to the casing toward and away from the piston, the inner face of the member being adjacent one end face of the piston and together with the latter forming a throttling throat communicating with said passage, a stop for limiting motion of the piston toward said wall portion, and means for adjusting the relative positions of said wall portion and said stop to thereby vary the width of the throat; a fluid connection between said one of said conduits, at a point in the conduit between the motor and the check valve, and the second port, a fluid connection between the sump and the first port, and a fluid connection between the third port and the other one of said conduits.

12. In combination with an abrasive wheel dressing element and a positive displacement motor for operating the same, a pair of conduits connected to the opposite sides of the motor, a reversing valve for alternately connecting the conduits respectively to sources of fluid at relative- 1y higher and lower pressures to thereby alternately reverse the motion of the motor; a check valve in one of said conduits to prevent the flow therethrough of fluid exhausting from the motor; a throttle valve having a casing, a piston movable in the casing, the casing having a chamber adjacent each of the two end faces of the piston, the piston having a passage therethrough extending from one end face thereof to a first port in the casing, the casing having second and third ports opening respectively into the chamber adjacent said one end face of the piston and the chamber adjacent the opposite end face of the piston, the first-mentioned chamber having a wall portion which together with the adjacent end face of the piston forms a throttling throat communicating with said passage, a stop for limiting motion of the piston toward said wall portion, and means for adjusting the relative positions of said wall portion and said stop to thereby vary the width of the throat; a fluid connection between said one of said conduits, at a point in the conduit between the motor and the check valve, and one of the first and second ports, a fluid connection between the source of fluid at lower pressure and the other of said first and second ports, and a fluid connection between the third port and the other one of said conduits.

13. In combination with an abrasive wheel dressing element and a positive displacement motor for operating the same, a pair of conduits connected to the opposite sides of the motor, a reversing valve for alternately connecting the conduits respectively to sources of fluid at relatively higher and lower pressures, to thereby alternately reverse the motion of the motor; a check valve in one of said conduits to prevent the flow therethrough of fluid exhausting from the motor; a throttle valve having a casing, a piston movable in the casing, the casing having a chamber adjacent each of the two end faces of the piston, the piston having a passage therethrough extending from one end face thereof to a first port in the casing, the casing having second and third ports opening respectively into the chamber adjacent said one end face of the piston and the chamber adjacent the opposite end face of the piston, the first-mentioned chamber having a wall portion which together with the adjacent end face of the piston forms a throttling throat communicating with said passage, and a stop for limiting motion of the piston toward said wall portion; a fluid connection between said one of said conduits, at a point in the conduit between the motor and the check valve, and one of the first and second ports, a fluid connection between the source of fluid at lower pressure and the other of said first'and second ports, and a fluid connection between the third port and the other one of said conduits.

14. In combination with an abrasive wheel dressing element and a positive displacement motor for operating the same, a sump, a pump for withdrawing fluid from the sump and discharging it under pressure, a pair of conduits connected to the opposite sides of the motor, a reversing valve to connect either of said conduits to the pump discharge and the other thereof to the sump, and a check valve' in one of said conduits to prevent the flow therethrough of fluid exhausting from the motor; a throttle valve having a casing, a piston movable in the casing, the casing having a chamber adjacent each of the two end faces of the piston, the piston having a passage therethrough extending from one end face thereof to a first port in the casing, the casing having second and third ports opening respectively into the chamber adjacent said one end face of the piston and the chamber adjacent the opposite end face of the piston, the first-mentioned chamber having a wall portion which together with the adjacent end face of the piston forms a throttling throat communicating with said passage, a stop for limiting motion of the piston toward said wall portion, and means for adjusting the relative positions of said wall portion and said stop to thereby vary the width of the throat; a fluid connection between said one of said conduits, at a point in the conduit between the motor and the check valve, and one of the first and second ports, a fluid connection between the sump and the other of said first and second ports, and a fluid connection between the third port and the other one of said conduits.

15. In combination with an abrasive wheel dressing element and a positive displacement motor for operating the same, a sump, a pump for withdrawing fluid from the sump and discharging it under pressure, a pair of conduits connected to the opposite sides of the motor, a reversing valve to connect either of said conduits to the pump discharge and the other thereof to the sump, and a check valve in one of said conduits to prevent the flow therethrough of fluid exhausting from the motor; a throttle valve having a casing, a piston movable in the casing, the casing having a chamber adjacent each of the two end faces of the piston, the piston having a passage therethrough extending from one end face thereof to a first port in the casing, the casing having second and third ports opening respectively into the chamber adjacent said one end face of the piston and the chamber adjacent the opposite end face of the piston, a member adjustable relative to the casing toward and away from the piston, the inner face of the member being adjacent one end face of the piston and together with the latter forming a throttling throat communicating with said passage, a stop for limiting motion of the piston toward said wall portion, and means for adjusting the relative positions of said wall portion and said stop to thereby vary the width of the throat; a fluid connection between said one of said conduits, at a point in the conduit between the motor and the check valve, and the second port, a fluid connection between the sump and the first port, and a fluid connection between the third port and the other one of said conduits.

16. A throttle valve having a passage therethrough for the fluid to be throttled, comprising a casing having inlet and outlet ports for said passage, first and second members movable within the casing, a stop for limiting the motion of the first member in the casing in one direction, said members having surfaces which together define a throttle throat in the passage when the first member is against the stop, a third port in the casing for admitting fluid against the first member to hold it against the stop or for exhausting fluid to permit the first member to move away from said stop, to separate said surfaces and thereby allow increased flow through the passage, and means for 9 adjusting the second member in the casing to vary the size of said throttle throat.

17. A fluid system having a passage extending between sources of fluid at different pressures, a throttle valve in the passage comprising a casing, first and second members movable within the casing, a stop on the casing for limiting the movement of the first member in one direction, said members having surfaces which together define a throttle opening in the passage when the first member is against the stop, means for adjusting the second member in the casing to adjust the size of the throttle opening, and the casing having a third port for admitting fluid against the first member, and the system further including means for connecting said third port either to a relatively high pressure source to move the first member against the stop or to a relatively low pressure source to allow the first member to move away from the stop under the pressure of fluid in said passage.

References Cited in the file of this patent UNITED STATES PATENTS 

